Rates of water exchange for two cobalt(II) heteropolyoxotungstate compounds in aqueous solution

Polyoxometalate ions are used as ligands in water-oxidation processes related to solar energy production. An important step in these reactions is the association and dissociation of water from the catalytic sites, the rates of which are unknown. Here we report the exchange rates of water ligated to Co(II) atoms in two polyoxotungstate sandwich molecules using the (17)O-NMR-based Swift-Connick method. The compounds were the [Co(4)(H(2)O)(2)(B-α-PW(9)O(34))(2)](10-) and the larger αββα-[Co(4)(H(2)O)(2)(P(2)W(15)O(56))(2)](16-) ions, each with two water molecules bound trans to one another in a Co(II) sandwich between the tungstate ligands. The clusters, in both solid and solution state, were characterized by a range of methods, including NMR, EPR, FT-IR, UV-Vis, and EXAFS spectroscopy, ESI-MS, single-crystal X-ray crystallography, and potentiometry. For [Co(4)(H(2)O)(2)(B-α-PW(9)O(34))(2)](10-) at pH 5.4, we estimate: k(298)=1.5(5)±0.3×10(6) s(-1), ΔH(≠)=39.8±0.4 kJ mol(-1), ΔS(≠)=+7.1±1.2 J mol(-1) K(-1) and ΔV(≠)=5.6 ±1.6 cm(3) mol(-1). For the Wells-Dawson sandwich cluster (αββα-[Co(4)(H(2)O)(2)(P(2)W(15)O(56))(2)](16-)) at pH 5.54, we find: k(298)=1.6(2)±0.3×10(6) s(-1), ΔH(≠)=27.6±0.4 kJ mol(-1) ΔS(≠)=-33±1.3 J mol(-1) K(-1) and ΔV(≠)=2.2±1.4 cm(3) mol(-1) at pH 5.2. The molecules are clearly stable and monospecific in slightly acidic solutions, but dissociate in strongly acidic solutions. This dissociation is detectable by EPR spectroscopy as S=3/2 Co(II) species (such as the [Co(H(2)O)(6)](2+) monomer ion) and by the significant reduction of the Co-Co vector in the XAS spectra.     

Density functional theory (DFT) investigation of the oxidative degradation of NaAsO2 via hydroxyl radical

Structural Chemistry - Arsenic is an environmentally ubiquitous health hazard due to its toxicity combined with its natural abundance and heavy industrial applications. Due to its role in...     

Nonequilibrium phase behavior during the random sequential adsorption of tethered hard disks

We simulate random sequential adsorption of tethered hard disks which undergo limited Monte Carlo surface diffusion to study kinetics and nonequilibrium phase behavior. Tethers allow the disks to move within a specified distance of their original adsorption locations, placing a control on the local entropy of each disk. As the surface coverage increases, systems with sufficiently long tethers form hexatic or crystalline lattices, while short tethers frustrate organization. Lattices form with surprisingly short tether lengths-on the order of one disk diameter.     

Physicochemical characterization of a new raw material obtained from leaves of <Emphasis Type="Italic">Syzygium cumini</Emphasis> (L.) Skeel (Myrtaceae)

The aim of this study was to determine the physicochemical characteristics of leaves of Syzygium cumini L. Skeels plant and characterize the extract of this plant by analytical techniques. Pharmacopeial methods of physicochemical analysis were used, including morphological characterization of the particle, thermal analysis, infrared spectroscopy and X-ray diffraction. The plant drug was presented as a coarse powder, within the appropriate Brazilian Pharmacopoeia parameters. The X-ray diffraction, optical microscopy and scanning electron microscopy suggested that the extract particles are amorphous and have irregular shapes, so that clusters of different sizes and morphologies are displayed. Thermal decomposition of the organic components in the sample started in a step that occurred between 151.64 and 209.27 °C with mass loss of 9.08 %, followed by another step with more significant mass loss (28.16 %). The infrared spectrum, in turn, showed many functional groups of compounds present in the lyophilized extract in different absorption bands. The results showed that the analytical techniques allowed us to characterize the physicochemical properties of the plant leaves, which may be useful in the production of new herbal medicines.     

Enantioselective Nickel-Catalyzed anti-Arylmetallative Cyclizations onto Acyclic Ketones

Domino reactions involving nickel-catalyzed additions of (hetero)arylboronic acids to alkynes, followed by cyclization of the alkenylnickel intermediates onto tethered acyclic ketones to give chiral tertiary-alcohol-containing products in high enantioselectivities, are described. The reversible E/Z isomerization of the alkenylnickel intermediates enables overall anti-arylmetallative cyclization to occur. The ring system of the products are substructures of certain diarylindolizidine alkaloids.     

Zintl‐Phase Sr3LiAs2H: Crystal Structure and Chemical Bonding Analysis by the Electron Localizability Approach

The compound Sr₃LiAs₂H was synthesized by reaction of elemental strontium, lithium, and arsenic, as well as LiH as hydrogen source. The crystal structure was determined by single‐crystal X‐ray diffraction: space group Pnma; Pearson symbol oP28; a = 12.0340(7), b = 4.4698(2), c = 12.5907(5) Å; V = 677.2(1) ų; R_F = 0.047 for 1021 reflections and with 36 parameters refined. The positions of the hydrogen atoms were first revealed by the electron localizability indicator and subsequently confirmed by crystal structure refinement. In the crystal structure of Sr₃LiAs₂H the metal atoms are arranged in a Gd₃NiSi₂‐type motif, whereas the hydrogen atoms are arranged in a distorted tetrahedral environment formed by strontium. The calculated band structure revealed that Sr₃LiAs₂H is a semiconductor, which is in agreement with its diamagnetic behavior. Thus, Sr₃LiAs₂H is considered as a (charge‐balanced) Zintl phase.     

Modeling and Uncertainty Quantification of Vapor Sorption and Diffusion in Heterogeneous Polymers

A high‐fidelity model of kinetic and equilibrium sorption and diffusion is developed and exercised. The gas‐diffusion model is coupled with a triple‐sorption mechanism: Henry’s law absorption, Langmuir adsorption, and pooling or clustering of molecules at higher partial pressures. Sorption experiments are conducted and span a range of relative humidities (0–95 %) and temperatures (30–60 °C). Kinetic and equilibrium sorption properties and effective diffusivity are determined by minimizing the absolute difference between measured and modeled uptakes. Uncertainty quantification and sensitivity analysis methods are described and exercised herein to demonstrate the capability of this modeling approach. Water uptake in silica‐filled and unfilled poly(dimethylsiloxane) networks is investigated; however, the model is versatile enough to be used with a wide range of materials and vapors.